FIGS. 12 and 13 show a DC-DC converter having a charging function in which a cell, such as a conventional fuel cell or solar cell is used as an input source, and a load Ro and a secondary cell B are connected in parallel to an output end of the DC-DC converter. The DC-DC converter having a charger shown in FIG. 12 has means for controlling an input voltage, which is lowered due to output impedance when electric power is supplied, so as to become constant, thereby realizing constant input of electric power in which when an input supply power Pin is smaller than an output supply power Pout, the input voltage becomes constant, and an output voltage becomes a dropping state, and means for controlling the output voltage so as to become constant, in which when the input supply power Pin is larger than the output supply power Pout, the input voltage increases, (for example, see to Patent Document 1 for the solar cell).
Furthermore, FIG. 13 shows a charger shown includes a charging circuit 20 is connected to the secondary cell B, and the charger has a unit that controls an input voltage, which is lowered due to output impedance when electric power is supplied, so as to be constant, thus realizing constant input of electric power. In addition, in the charger circuit the input power Pin is set to be larger than the output power Pout, and a constant voltage control circuit and a constant current control circuit are provided in order to stably perform the charging of a current into the secondary cell B.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-341699